This paper reports on the case of selection, due to influence of bad weather on nestling number, acting through different pathways on breeding date and morphological traits in breeding Great Tits. Multiple regression of relative fitness on tarsus length, bill height and forearm length, revealed significant phenotypic selection on male forearm length and female bill height. However, nestling mortality during the bad weather depended on their age, which suggested breeding date as a focal trait for selection. The multiple regression analysis of relative fitness on both body traits and hatching date shows that in males only breeding date was significantly related to fitness, and selection on forearm resulted from correlation of forearm with hatching date. A similar analysis in females shows that both bill height and timing of hatching were equally related to fitness, therefore in this sex both bill height and date of hatching were focal traits for selection.
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Vol. 37 • No. 2